1. Field of the Invention
The present invention relates to a magnetic recording medium of paint type and, in particular, to a magnetic recording medium having an excellent dispersing ability of a magnetic powder.
2. Description of the Prior Art
Magnetic recording medium are widely used in the forms of audio tapes, video tapes, backup data cartridges. floppy disks and the like. For these magnetic recording media, discussions are going on so as to obtain a higher density such as shorting the recording wave length, requiring an improvement of electromagnetic conversion property.
In the paint type magnetic recording medium, in order to attain a high density recording, there are considered various techniques such as a use of a ferromagnetic alloy powder as the ferromagnetic powder, refining of the ferromagnetic powder, increase of the coercive force of the ferromagnetic powder and uniform distribution thereof, and a mirror finished surface of a magnetic layer. At present, the magnetic material has been significantly improved and it is possible to obtain a ferromagnetic powder having a saturation magnetization exceeding 140 Am.sup.2 /kg and a ferromagnetic powder having a longer axis length of 0.1 .mu.m or below. Moreover, there exists a ferromagnetic powder having a coercive force exceeding 160 kA/m with a uniform particle size distribution reflected in the coercive force. Thus, a remarkable development can be seen in this field.
It is necessary to uniformly distribute the ferromagnetic powder in a magnetic paint so that the magnetic recording medium can exhibit a preferable electromagnetic conversion property. Actually, however, it becomes more and more difficult to uniformly distribute a ferromagnetic powder of a smaller particle size which satisfies the aforementioned conditions.
In order to cope with such a circumstance, it is considered to increase a time required for kneading or dispersion when preparing a magnetic paint. However, this also causes deterioration of the ferromagnetic powder, decreasing the production efficiency.
On the other hand, it has also been tried that a bonding agent is made to contain a dispersing agent having a particular functional group, so as to increase the interaction with the ferromagnetic powder and improve the dispersibility. However, the dispersing agent presently available cannot sufficiently disperse a fine ferromagnetic powder which has been developed for a high-density recording.